Solid oxide fuel cells (SOFCs) include a porous cathode, a porous anode, and a cell structure including a dense solid electrolyte layer provided therebetween, an oxidant channel to supply an oxidant to the cathode, and a fuel channel to supply a fuel to the anode. In currently available SOFCs, an oxygen-ion conductive metal oxide such as yttria-stabilized zirconia (YSZ) as a typical example is used as a solid electrolyte. To improve power generation characteristics of SOFCs, the thickness of the solid electrolyte layer is preferably minimized. Thus, an electrolyte layer-electrode assembly including a solid electrolyte layer arranged on a cathode or an anode having enhanced mechanical strength is used in some cases. Such an assembly is referred to as a cathode-supported solid electrolyte layer or an anode-supported solid electrolyte layer.
The anode-supported solid electrolyte layer is generally produced by forming a coating film containing a solid electrolyte on a surface of the green body of a mixture of a nickel component (such as NiO) serving as a hydrogen dissociation catalyst and a solid electrolyte serving as an oxygen-ion conductor and sintering (co-sintering) the resulting component. The anode produced in this way is initially dense. However, when the anode is used in an SOFC, NiO is reduced to Ni by hydrogen serving as fuel supplied to the anode, and the anode is changed into a porous anode by volumetric shrinkage occurring simultaneously with the reduction.
PTL 1 reports that in an anode-supported solid electrolyte layer containing an oxide ionic conductor such as YSZ, the size of pores in the anode is adjusted from the viewpoint of improving the economic efficiency of an SOFC for large-scale power generation and stabilizing a solid electrolyte layer.
PTL 2 reports that in an anode-supported solid electrolyte layer, porosity is adjusted in such a manner that the anode has sufficient strength and a fuel gas flows easily. In PTL 2, a solid electrolyte such as YSZ, which is an ion-conductive oxide, is used for the anode, and a pore-forming material is used in order to form the porous anode prior to the reduction of NiO.
The operating temperature of an SOFC containing oxygen-ion conductive YSZ serving as an electrolyte is about 750° C. to about 1,000° C. at present, and an expensive oxidation-resistant material is used in a component. Thus, there is a need for the development of an SOFC that operates an intermediate temperature range of 400° C. to 600° C., in which inexpensive general-purpose stainless steel can be used. Perovskite oxides such as BaCe0.8Y0.2O2.9 (BCY) and BaZr0.8Y0.2O2.9 (BZY) exhibit high proton conductivity in the intermediate temperature range and thus are promising solid electrolytes for intermediate-temperature type fuel cells.